True: the time of flight of the two balls is the same
Explanation:
The ball that is dropped is in free fall, so its time of flight can be determined by using the suvat equation

where
is the vertical displacement, which is the height from which it was dropped
u = 0 is the initial vertical velocity
is the acceleration of gravity
t is the time of flight
Solving for t,

The ball thrown horizontally follows a projectile motion, which consists of two independent motions:
- A uniform motion (constant velocity) along the horizontal direction
- A uniformly accelerated motion, with constant acceleration (acceleration of gravity) in the downward direction
However, the time of flight is completely determined by the vertical motion only, therefore using the equation:

where u = 0 is the initial vertical velocity of the ball (which is zero, since it is thrown horizontally). Since the ball is thrown from the same height, its time of flight is still

So, the two balls reach the grund at the same time.
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